In situ combustion of lignite



May 15, 1962 F. E. FREY ET AL IN SITU COMBUSTION OF LIGNITE Filed Aug. 51, 1959 OVERBURDEN INVENTORS F. E. FREY E W. CRAWFORD BY J. W. MARX A T TORNE VS United States Patent Ofi ice 3,034,580 Patented May 15, 1962 3,034,580 IN SITU COMBUSTION F LIGNITE Frederick E. Frey, Francis W. Crawford, and John W.

Marx, Bartlesville, Okla, assignors to Phillips Petroleum Company, a corporation of Delaware Filed Aug. 31, 1959, Ser. No. 836,938 I 5 Claims. (Cl. 16611) This invention relates'to a process for the production of combustible fluids from lignite by autogenous in situ combustion.

It has been proposed to burn subterranean fuel deposits such as oil sands, tar sands, oil shales, and coal, including lignite, in situ by both inverse and direct air injection.

We have found that the in situ combustion of lignite by igniting the same and injecting air into the ignited area sutficiently burns the lignite but because of its low ash content, the combustion forms cavities in the bed and, thereafter, combustion occurs as a flame in which most of the useful products are burned. Lignite from the Baukol- Noonan Mine of North Dakota was obtained and a number of combustion runs were made with this lignite. The lignite was packed into both stainless steel and Vycor tubes and ignition was effected at one end; The combustion front was driven thru the packedbed by both direct and indirect air injection in separate runs. The combustion temperature was slightly above 1000 F. at an air velocity of 300 ft./hr. In the tests the amounts of useful substances such as hydrogen, carbon monoxide, and hydrocarbons produced'were so small as to be considered substantially negligible. This was attributed to the fact that the combustion formed cavities, clearly visible thru the transparent Vycor tubes, which allowed the lignite to burn as a flame thereby consuming substantially all of the fuel. The produced gases from the combustion tests utilizing concurrent flame propogation was 40.3 B.t.u./ cu. ft. and for counter current flame propagation the produced gases had a heating value of 36.1 B.-t.u./cu. ft.

We have found that many lignite deposits can be produced by in situ combustion so as to recover a significant proportion of the fuel in the deposit as valuable fuel gases and liquids. Lignites contain from 5 to about 30 weight percent of oxygen and those deposits containing at least weight percent of oxygen can be economically produced by autogenous in situ combustion.

Accordingly, it is an object of the invention to provide an improved process for producing lignite deposits by in situ combustion. Another object is to provide a process for producing a lignite deposit by in situ combustion which avoids consuming all of the fuel in the deposit and pro duces a substantial proportion thereof as valuable fluid fuels. It is also an object of the invention to provide a process for the in situ combustion of lignite which avoids burning of the lignite by flame combustion. Other objects will become apparent upon consideration of the accompanying disclosure.

A broad aspect of the invention comprises igniting a lignite deposit or stratum in a selected area so as to establish a combustion zone and continuing the combustion by excluding extraneous oxygen therefrom and allowing the combustion to continue with the oxygen originally contained in the deposit and recovering the fluids produced by the combustion. This process is applicable to a lignite deposit containing about 10 weight percent or more oxy zen.

Ignition of the lignite is eifected by drilling a borehole or well into the deposit, preferably to the bottom of the deposit, and igniting the lignite in the wall of the well by any suitable method. An effective method for the ignition step comprises extending a tubing string to the bottom of the well and packing particulate charcoal around the tubing string to the top of the lignite stratum. The charcoal is then ignited and air or other combustion-supporting gas, such as oxygen or oxygen-enriched air, is injected into the charcoal either thru the tubing string or thru the annulus around the tubing so as to burn the charcoal and heat the lignite to combustion temperature. After consumption of the charcoal, the flow of air is cut off and the lignite burns with the oxygen originally present therein. The combustion products pass into the Well and may be recovered therefrom either thru the annulus or the tubing. In order to facilitate withdrawal or recovery of the produced gases during the autogenous combustion, a flushing gas devoid of free 0 may be injected either thru the tubing or thru the annulus so as to flush the produced gases thru the other. Combustiongas is a suitable flushing gas, although any other substantially inert gas may be utilized.

In order to illustrate the invention reference is made to the schematic drawing which is an elevation in partial section thru a lignite stratum showing an arrangement of apparatus and wells for effecting the invention. A lignite stratum 10 is penetrated by an ignition well 12 and an offset well 14 which are provided with tubing strings 16 and 18, respectively, and with casings 20 and 22, respectively. Casing 29 is provided with line 24 for injection of air and nonoxidizing gas. the same purpose.

It is to be understood that ignition well 20 may be surrounded by a ring of olfset wells 14 or it may be one of a line of ignition wells flanked on either side by parallel lines of offset wells 14. v

In operation, the lignite around Well 12 is ignited in conventional manner by bringing the lignite to ignition temperature and injecting air thru line 24 and the tubingcasing annulus. After ignition is established in a substantial area of the stratum adjacent well 12, the flow of air is terminated and a relatively inert flushing gas such as' combustion gas is injected thru either tubing string 16 or line 24 so as to flush out the hydrocarbons produced by the autogenous combustion. It is also feasible to produce the hydrocarbon gases and liquids resulting from the combustion thru offset well or wells 14 either thru line 26 or thru tubing string 18. In accordance with another mode of operation, the flushing gas is injected thru the ofiset well 14 via either line 26 or tubing string 18 so as to flush the produced hydrocarbons into well 12 from which they are produced via tubing string 16 or line 24.

During drying out of the stratum either tubing string 16 or 18 may be continued to the bottom of the hole to remove produced water or a pump may be utilized for this purpose.

In some lignite strata or deposits the moisture conditions therein are not conducive to autogenous in situ combustion. In such deposits it is necessary to drill an oifset well in the deposit spaced a short distance from the ignition well and dry out the intervening lignite. A preferred practice comprises drilling a ring of wells around the ignition well and injecting air thru the offset wells so as to drive water from the intervening stratum into the ignition well from which it is forced or pumped so as to dry out the stratum. It is also feasible to inject air thru .the ignition well and recover the flushed out water thru the wells in the ring. After the flushing step, ignition is initiated around the ignition well in the aforesaid manner, or in any other suitable manner, and produced gases are recovered therefrom as the combustion proceeds. Heating Line 26 connects with casing 22 for i tial proportion of the lignite deposit as valuable fluids utilizable as fuel and for other purposes.

Combustion catalysts may be utilized as an aid in initiating combustion. Iron-containing catalysts appear to be the most efiective.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative de-,

tails disclosed are not to be construed as imposing unnecessary limitations on the invention.

We claim: 7

1. A process for producing fuel gases and liquids -from a lignite stratum containing water and at least about 10 weight percent oxygen by autogenous in situ combustion which comprises providing an ignition well and at least one offset well therein; injecting 'air thru one of said wells and passing same thru the lignite to the other well to drive moisture therefrom and dry out the intervening.

2. A process for producing fuel gases and liquids from a lignite stratum containing at least 10 weight percent oxygen by autogenous in situ combustion which comprises providing a well extending from ground level into said stratum and a tubing string in said Well forming an annulus with the Wall of said well; igniting the lignite adjacent said well by heating same to ignition temperature and injecting combustion-supporting gas thru one of said annulus and said tubing so as to contact the heated lignite with said gas and establish a combustion zone in said lignite; after ignition is efiected, terminating the flow of said gas and substituting therefor an O -free flushing gas, to remove from said Well fluids produced by the combustion.

3. A process for producing fuel gases and liquids from a lignite stratum containing at least 10 weight percent oxygen by autogenous in situ combustion which comprises providing a well extending from ground level into said stratum; igniting the lignite around the Wall of said well to establisha combustion zone therein by heating said wall to ignition temperature and contacting the heated wall with combustion-supporting gas; terminating the contacting with'saidgas and passing a stream of O -free flushing gas in contact with the combustion Zone to flush out fluids produced by said combustion; and recovering flushed fluids produced by the combustion.

4. The process of claim 3 wherein said flushing gas is injected thru said offset Well and fluids are produced thl'u said ignition well. 7

5. The process of claim 3 wherein said flushing gas is injected thru said ignition well and fluids are produced thru said offset well.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Mining Engineers Handbook, Peele, 3rd ed., 1941, vol. I, pages 230.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,034,580 May 15, 1962 Frederick E. Frey et. a1.

It is hereby certified that error appears in the above numbered pat ent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, lines 19 and 22, for the claim reference numeral "3", each occurrence, read l Signed and sealed this 30th day of October 1962.

(SEAL) Attest:

DAVID L. LADD ERNEST W. SWIDER Commissioner of Patents Attesting Officer 

2. A PROCESS FOR PRODUCING FUEL GASED AND LIQUIDS FROM A LIGNITE STRATUM CONTAINING AT LEAST 10 WEIGHT PERCENT OXYGEN AUTOGENOUS IN SITU COMBUSTION WHICH COMPRISES PROVIDING A WALL EXTENDING FROM GROUND LEVEL INTO SAID STRATUM AND A TUBING STRING IN SAID WELL FORMING AN ANNULUS WITH THE WALL OF SAID WELL; IGNITE THE LIGNITE ADJACENT SAID WELL BY HEATING SAME TO IGNITION TEMPERATURE AND INJECTING COMBUSTION-SUPPORTING GAS THRU ONE OF SAID ANNULUS AND SAID TUBING SO AS TO CONTACT THE HEATED LIGNITED WITH SAID GAS AND ESTABLISH A COMBUSTION ZONE, IN SAID LIGNITE; AFTER INGINITION IS EFFECTED, TERMINATING THE FLOW OF SAID GAS AND SUBMITTING THEREFOR AN O2-FREE FLUSHING GAS TO REMOVE FROM SAID WELL FLUIDS PRODUCE BY THE COMBUSTION. 